plot in velocity relative to LSR about blimpy

[telegraphic]

This should be doable, will require computation of LSRK and then conversion of frequency axis to velocity.

Blimpy currently has a compute_lsrk() function that requires PySLALIB, and reports back a LSRK value. This code (should) do the same thing, but within astropy:

# Code modified from: https://gist.github.com/StuartLittlefair/5aaf476c5d7b52d20aa9544cfaa936a1
from astropy.time import Time
from astropy.coordinates import SkyCoord, solar_system, EarthLocation, ICRS, UnitSphericalRepresentation, CartesianRepresentation
from astropy import units as u
from astropy import constants as const
from astropy.coordinates import EarthLocation
from astropy.coordinates import Angle

def velcorr(time, skycoord, location=None):
    """Barycentric velocity correction.
    
    Uses the ephemeris set with  ``astropy.coordinates.solar_system_ephemeris.set`` for corrections. 
    For more information see `~astropy.coordinates.solar_system_ephemeris`.
    
    Parameters
    ----------
    time : `~astropy.time.Time`
      The time of observation.
    skycoord: `~astropy.coordinates.SkyCoord`
      The sky location to calculate the correction for.
    location: `~astropy.coordinates.EarthLocation`, optional
      The location of the observatory to calculate the correction for.
      If no location is given, the ``location`` attribute of the Time
      object is used
      
    Returns
    -------
    vel_corr : `~astropy.units.Quantity`
      The velocity correction to convert to Barycentric velocities. Should be added to the original
      velocity.
    """
    
    if location is None:
        if time.location is None:
            raise ValueError('An EarthLocation needs to be set or passed '
                             'in to calculate bary- or heliocentric '
                             'corrections')
        location = time.location
      
    # ensure sky location is ICRS compatible
    if not skycoord.is_transformable_to(ICRS()):
        raise ValueError("Given skycoord is not transformable to the ICRS")
    
    ep, ev = solar_system.get_body_barycentric_posvel('earth', t) # ICRS position and velocity of Earth's geocenter
    op, ov = location.get_gcrs_posvel(t) # GCRS position and velocity of observatory
    # ICRS and GCRS are axes-aligned. Can add the velocities
    velocity = ev + ov # relies on PR5434 being merged
    
    # get unit ICRS vector in direction of SkyCoord
    sc_cartesian = skycoord.icrs.represent_as(UnitSphericalRepresentation).represent_as(CartesianRepresentation)
    return sc_cartesian.dot(velocity).to(u.km/u.s) # similarly requires PR5434



pks = EarthLocation(Angle(148.2614, unit='degree'), Angle(-32.9994, unit='degree'), 414.80 * u.m)
t   = Time(b.axes.time[0], format='mjd')
ra  = b.axes.coordinates['ra'][0]
dec = b.axes.coordinates['dec'][0]
radec = SkyCoord(ra, dec, 'icrs', unit=('hourangle', 'degree'))

print velcorr(t, radec, location=pks)